It is now generally the case that storage systems need to be available constantly. This means that in a storage area network (SAN) the individual storage disks or devices need to be hot-swappable in the event of failure in order to avoid having to take the storage off-line. A high proportion of SANs use SCSI or Fibrechannel disks which are hot-swappable but they are expensive.
A cheaper disk type is IDE and with the advent of iSCSI, aggregation of IDE based storage devices into a SAN is attractive. Hot swap capability for IDE drives is being developed but this imposes an expense and space restriction, including having more expensive mechanical components with slide rails.
It is desirable to be able to use cheaper forms of disk storage, without hot-swap capability, but to be able to replace failed disks without down time.